1. Field of the Invention
This invention relates to a process of forming thick layers of photoresist upon a substrate by the spin resist technique. More particularly the invention relates to a novel method of making fine line conductors suitable for use in the manufacture of microcircuits for the connection and interconnection of monolithic circuits as well as for the manufacture of windings in integrated recording heads. Further, the invention relates to a novel method of applying a uniform photoresist film and a method of forming fine line patterns therefrom.
2. Description of the Prior Art
With monolithic circuitry in which a large number of circuits may be formed over a small area, the connection and interconnection of such circuits have caused serious problems. Many such circuits require relatively high currents for proper operation, and yet the space available for the carrying of such currents to monolithic chips is severely limited. Prior art devices have proposed multi-layer packages for the fabrication of the required number of circuit lines with a cross-sectional area sufficiently large to carry the current needed. The fabrication of such multilayer circuits often result in low yields since it is a multistep process in which the layers must be built up, properly registered, and interconnected.
The manufacture of fine-lines, in the order of microns, using etching techniques is well known in the semiconductor manufacturing art. The method consists of the evaporation of a thin film of conductive metal followed by the application of a thin film of photoresist. The film is exposed and developed, after which the metal may be etched. Very thin photoresist films, usually of less than a micron in thickness, are deposited for such applications since all that is required is that they withstand the metal etchant. Because of the thinness of the photoresist film, a relatively large percentage variation in thickness over the film can be tolerated, since such percentage variation usually amounts to little actual thickness variation and it is the actual thickness variation which caues under- or over-exposure of portions of the resist. Although the lines produced by such an etching process may be of micron widths, the thickness of the lines is usually of an order of magnitude thinner so as to limit the amount of undercutting inherent in the etching process, thus obtaining better control of the cross-sectional area of the conductor. Because the thickness of these lines is relatively thin in comparison to their width, the cross-sectional area of the lines is minimized, thereby minimizing their current carrying capacity. Thus, the lines produced by such a process, although suitable for the manufacture of circuits on the semiconductor chip itself, do not have the current carrying capacity necessary for their use in the connection and packaging of such chips. The deposition of thicker metal films for use in such an etching process is of no advantage since it requires the formation of wider lines so that the inherent undercutting does not cause lifting of the photoresist film from the substrate with trapezoidal or even triangular sections formed during the etching process.
Another application for the use of ultra-fine line circuitry with maximum current carrying capacity is in the manufacture of miniaturized integrated magnetic recording heads as discussed by Valstyn, "Integrated Head Developments", Annuls New York Academy of Sciences, Vol. 189, p. 191. Such devices require ultra-narrow conductors with maximum current carrying capacity so as to obtain a large number of conductor turns in a small area in order to produce a large quantity of magnetic flux. Prior art miniaturized heads have contemplated the use of copper conductors 250 microns wide by 6 microns thick. Such dimensions, although yielding an overall cross-sectional area larger than that contemplated by this invention, limit the number of conductor turns, thereby limiting the magnetic flux produced. It can be seen that by increasing t's thickness to width ratio, or aspect ratio, from that which has heretofore been obtained for such devices, 0.54, the number of turns may be increased without affecting the current carrying capacity, thereby increasing the total magnetic flux produced. As previously discussed, because of the inherent undercutting in the etching process, there is a maximum aspect ratio that can be obtained from such a process. In a typical etching process aspect ratios of greater than 0.4 result in severe undercutting, often causing complete disappearance of the line produced. For example, it is impossible using the conventional etching techniques to form 4 micron wide lines on 8 micron centers in a metal film whose thickness is greater than 1.8 microns.
Because of the limited aspect ratio obtainable in an etching process, it has been proposed to employ an additive metal plating process to achieve thick fine line circuitry with large current carrying capacity in densely packed areas. Heretofore, it has been impossible to produce ultra-fine line circuits with widths of less than 5 microns as contemplated by this invention and still build up the required plating thickness to yield large aspect ratios. In order to produce densely packed, fine line circuits, such as 5 micron wide lines on 8 micron centers, as contemplated by this invention, extremely accurate resolution during exposure is required. Underexposure of a positive photoresist, i.e., a resist which becomes soluble upon exposure to ultraviolet radiation, will result in incomplete removal of the resist and subsequent nonplating of the circuit lines in those areas. The circuit lines produced due to such underexposure will be narrower than desired, often resulting in the breakage of a line and a corresponding open circuit. Overexposure of a positive photoresist results in wider lines than desired, and in the case of densely packed circuitry such as the 3 microns spaces contemplated by this invention, shorting between lines may result. Proper exposure and development of a photoresist pattern, so as to faithfully reproduce the mask pattern, becomes critical in the manufacture of such fine line circuitry. Proper exposure can be obtained only if the photoresist is of uniform thickness. Unlike the previously discussed semiconductor etching process, wherein the photoresist is applied as an extremely thin film to minimize the effect of nonuniformity, the resist in a plating process must be applied to a thickness equal to the thickness of the plating desired. Attempts to obtain reliable fine line circuitry with aspect ratios greater than 0.4 have heretofore failed on account of the non-uniformity of the application of the resist, and subsequent incorrect development and exposure.
Also, additional problems arise where the resist must be coated upon a non-uniform by contoured substrate. Such may be the case where the insulation upon which the conductive pattern is to be deposited conforms to the contour of underlying conductors as in a multilayer circuit or multiturn miniaturized recording head. With such devices it is impossible to obtain completely uniform resist coatings since some excess will always flow to the valleys from the peaks. Completely uniform exposure is also impossible since the peaks and valleys vary in distance from the light source. Because of such inherent non-uniformity, resist in the valley regions will be under-exposed as compared with the peak regions. Such under-exposure, as previously discussed, results in narrowing of the circuit lines.